This invention relates generally to hydraulically operated machines and, in particular, to such machines having multiple tandem articulated members which move in unison in order to produce a desired movement of one of the members. The invention is particularly adapted for use with hydraulically controlled construction equipment, such as hydraulic excavators.
Hydraulically controlled excavators typically include a body, or cab, which is pivotally mounted to a truck or track unit, to rotate about a vertical pivot, a boom pivotally mounted to the cab, a stick, or arm, pivotally mounted at the distal end of the boom, and a bucket pivotally mounted to the distal end of the arm. Hydraulic cylinders attached between the cab and the boom control pivotal movement of the boom. Hydraulic cylinders between the boom and arm control pivotal movement of the arm and hydraulic cylinders between the arm and bucket control pivotal movement of the bucket. In manually controlled excavators, an operator has separate control mechanisms, such as joysticks or levers, for operating all aforementioned cylinders in order to attempt to move a cutting edge of the bucket in a desired fashion. The difficulty with such manual mechanisms is that it is difficult for the operator to move the bucket cutting edge along a level plane, especially when excavating below ground, in order to, for example, prepare the site for footings or sewer pipe. In order to overcome such difficulty, it is known to assist the operator by providing an automatic control system which causes the excavator to move the cutting edge of the bucket along a fixed plane. One such control is disclosed in U.S. Pat. No. 4,829,418 issued May 9, 1989, to Edward G. Nielsen and Timothy E. Steenwyk for an APPARATUS AND METHOD FOR CONTROLLING A HYDRAULIC EXCAVATOR, which is commonly assigned with the present application. The control in the Nielsen et al. patent utilizes a rotating laser beacon set up at the construction site at the desired grade the excavator is to excavate to. A laser sensor is mounted to one of the moveable members, such as the stick, so as to pass through the laser beam during excavation. With a desired depth relative to the height of the laser plane entered into a control panel by the operator, the control system causes the cutting edge of the bucket to maintain the desired depth with respect to the rotating laser plane. Such control system allows the operator to maintain control over movement of the stick and/or bucket with the control system controlling movement of the boom. The boom is controlled in a manner which maintains the cutting edge of the bucket at the desired depth notwithstanding the motion of the bucket and stick.
One difficulty in automatically controlling the movement of the boom of the excavator in response to the movement of the stick by the operator is that delays accompanying the response of the hydraulic system to commands made upon the system make it difficult for the control system in a manner which accurately maintains the cutting edge of the bucket at the desired depth under all conditions.